Wave soldering in a low oxygen-containing atmosphere has demonstrated improved results over wave soldering in air or other high oxygen content atmospheres.
The presence of appreciable quantities of inert gas such as nitrogen gas improves solderability and reduces dross. More specifically, nitrogen gas improves the ability of solder to penetrate the clearance or joint between the component to be soldered to a substrate and the substrate itself (i.e. improved wicking or capillary action). To obtain maximum benefit from the introduction of nitrogen gas, it is important that the nitrogen gas be available at or near the apex of the solder wave where contact is made between the substrate and the solder.
The wicking action of the solder can also be improved if the solder wave undergoes some vibrational movement during the soldering operation. A slight vibrational motion is desired to avoid the presence of trapped gases in the joint. On the other hand, excessive vibrational movement will cause splattering of the solder onto the substrate resulting in poor solder connections and possibly defective products.
It is desirable to insure that the substrate to be soldered is as free of oxides as possible. In this regard it is helpful if the atmosphere in which the substrate is pretreated (e.g. preheated) and soldered be controlled so as to avoid the presence of excessive amounts of oxygen.
It is also desirable to prevent the formation of dross which results from the oxidation of the molten metal. Accordingly, it has been proposed to enclose the entire soldering operation from all or a portion of the preheating stage to the actual soldering stage in a hood as disclosed in U.S. patent application Ser. No. 08/067,764 filed May 26, 1993.
The three principal areas of dross formation in proximity to the solder wave are at the top or apex of the solder wave, where the wave descends from the apex, and near the walls of the solder pot.
There have been efforts to conduct wave soldering in a protective atmosphere which is generally defined as having an oxygen content of less than 10% by volume. Reducing the amount of oxygen in vicinity of the soldering operation results in less dross. Traditionally, the protective atmosphere is created by enclosing the wave soldering area in a protective hood and then filling the space beneath the protective hood with an inert gas, such as nitrogen gas.
The cost of supplying nitrogen gas on a continuous basis to the entire space beneath the soldering hood significantly increases the cost of wave soldering. Recently, there has been an attempt to provide a localized blanket of inert gas at the junction of the substrate to be soldered and the solder wave so as to minimize the amount of nitrogen gas which is used to generate a viable protective atmosphere. John H. Gileta et al., U.S. Pat. No. 5,203,489, disclose a cover or shroud which at least partially covers the solder reservoir. Above the surface of the solder in the solder reservoir and to the side of the solder wave, there is positioned at least one nozzle for injecting an inert gas. The shroud forces the injected nitrogen gas to flow along the surface of the solder wave up to the point of contact of the solder wave and the substrate to be soldered.
While such systems may be effective in minimizing dross where the solder wave descends, optimal protection against dross is not obtained at the apex of the wave and near the walls of the solder pot.
It would be desirable to provide a method and apparatus for generating a localized low oxygen-containing atmosphere in which the atmosphere is controlled and the amount of inert gas employed minimized. In addition, it would be desirable to provide a wave soldering process in which dross in all principal areas of dross formation, including those in proximity to the solder wave is effectively reduced.